1. Field of the Invention
The present invention relates to monolithic visible- or solar-blind photodetectors with resolved sensitivities in the UV and IR bands. Methods for fabrication of the photodetectors and a method for decreasing sensitivity of the integrated structure to visible or solar radiation are also provided.
2. Description of Related Art
Solid-state optical detectors based on semiconductor materials have replaced photoemissive devices in a wide variety of both commercial and military applications due to their broad spectral responsivity, excellent linearity, high quantum efficiency, high dynamic range of operation, and possibility of large-format image arrays. The spectral range of most semiconductor-based optical detectors is determined by optical absorption in the active semiconductor material layer at energies above the semiconductor band gap, the cutoff wavelength. In such terms narrow-band gap semiconductors, such as II-VI compounds like HgCdTe, are suitable for infrared detection, Si and some III-V compounds are suitable for detection in the visible and near infrared range, and wide band gap semiconductor materials, such as diamond, SiC, and III-Nitrides, are superior for applications in the UV range.
Several military and industrial applications require simultaneous (or at least spatially synchronized) detection of optical emissions in different spectral regions. A large number of various objects, such as, for example, fires, jet or rocket nozzles, hot filaments, stellar luminaries, electrical arcs, and lightning produce optical emissions ranging from ultraviolet to IR. Such emissions can be detected over the wide-range of ambient light background only by fast multi-range optical detectors allowing time-resolved measurements in particular optical bands. As a result, not only the spectral range, but also the detector speed and spatial resolution and alignment become critical for fast fire-detection with high false-alarm immunity. Currently used photomultiplier tubes (PMTs) have high sensitivity, but are bulky, require high voltage operation, and have low mechanical and temperature strength. Some recently developed flame detectors are composed of discrete UV and IR solid-state components in one housing, but these devices sustain temperatures only up to 125° C., and are not capable of detecting multi-band optical signals with high spatial resolution.
To date significant progress has been made in the development of UV detectors based on wide band gap materials. Several attempts to develop UV detector structures on diamond were made by 1996, but lack of high quality layers and insufficient doping levels did not result in practical devices. Visible-blind UV photodetectors have been fabricated on silicon carbide (SiC) substrates, but the technology is relatively immature due to the absence of high quality large area substrates until recent years.
Group III-nitride materials are superior for advanced UV detector fabrication due to their wider direct band gap and high thermal, chemical, mechanical, and radiation tolerance. A large amount of research by several groups has been dedicated to the development of UV detectors based on GaN, GaN/AlGaN, and AlGaN. Currently attracting most interest are AlGaN-based structures since they can provide detection in the very important UV range of 240-280 nm, which corresponds to the optical window where solar radiation is significantly absorbed by the ozone layer. Research and development performed by several groups indicate that effective optical emission and detection can be achieved in a wide spectral band ranging from 200 to 1770 nm. This would allow integrated nitride only-based devices working in separate bands (including UV and IR) for the entire referenced range.
In the area of IR detection, the conventional HgCdTe- and InSb-based detectors display high quantum efficiencies but are difficult to integrate into large arrays. Detectors based on heterointernal photoemission (HIP) in GexSi1-x/Si heterojunctions have demonstrated excellent opportunities for integration on Si wafers at sufficient sensitivities in the infrared range of 1-12 μm. Large area SiGe-based HIP photodetector arrays of 400×400 pixels have been available for close to ten years. Schottky barrier photodetectors based on metal silicides formed on silicon also allow extending the sensitivity to the longer IR (>than 1.1 μm) range.
The Radio Frequency Molecular Beam Epitaxy (RF MBE) method used for nitride material growth allows fabrication of multilayer structures that incorporate binary, ternary, or even quaternary nitride compounds with a precise control over the layer thickness, chemical composition, crystalline quality, and doping during a single-process growth on commercial sapphire or silicon substrates. Growth of III nitrides on Si wafers takes advantage of both the commercial and technological benefits offered by the well-commercialized silicon technology and the existing low cost electronic and optical IR devices. Device-quality GaN layers grown on silicon wafers have been demonstrated by several groups. Additional benefits for employment of silicon in the present invention come from its optical properties providing the ability to detect and block optical emissions in the near IR and visible ranges, respectively.
A multi-spectral infrared photodetector and imager is disclosed in U.S. Pat. No. 6,897,447 B2. Two or more different bands of IR radiation are detected by a diffractive resonant optical cavity. U.S. Pat. No. 6,049,116 also teaches a device and fabrication method for a two-color IR detector
What is needed is a miniature, chip-based dual-color high-temperature visible- or solar-blind photodetector that will allow for fast and efficient detection of optical emission in UV and IR bands using sensors in close proximity, so as to produce high spatial- and temporal-resolution signals over a wide range of operating temperatures. Such sensors should allow fabrication of multi-pixel focal arrays for dual-band visible- or solar-blind cameras, which can be used not only for fire/flame detection and imaging, but also for various space- and military-related applications that involve object/target recognition.